docs/manual: Document the variable $(PKG)_DL_OPTS

This adds a description of the optional variable $(PKG)_DL_OPTS. When it
is set, this option passes additional options to the downloader.

Signed-off-by: Romain Perier <romain.perier@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/adding-packages-generic.txt

@@ -250,6 +250,13 @@ information is (assuming the package name is +libfoo+) :
     +LIBFOO_SITE=/opt/software/libfoo.tar.gz+ +
     +LIBFOO_SITE=$(TOPDIR)/../src/libfoo+
 
+* +LIBFOO_DL_OPTS+ is a space-separated list of additional options to
+  pass to the downloader. Useful for retrieving documents with
+  server-side checking for user logins and passwords, or to use a proxy.
+  All download methods valid for +LIBFOO_SITE_METHOD+ are supported;
+  valid options depend on the download method (consult the man page
+  for the respective download utilities).
+
 * +LIBFOO_EXTRA_DOWNLOADS+ is a space-separated list of additional
   files that Buildroot should download. If an entry contains +://+
   then Buildroot will assume it is a complete URL and will download